Direction finding system



Patented June 10, 1947 DIRECTION FINDING SYSTEM Clarence W. Hansen, PortJelierson, N. Y., assignor 'to Radio Corporation of America, acorporation of Delaware Application September 30, 1942, Serial INo.460,297

Claims. 1

This invention relates to radio direction finding, and more particularlytoa system wherein bearing indications are produced with `maximum,signal pickup, rather than with minimum -or zero signal pickup.

Direction iinders ordinarily Aoperate-by virtue of aldirective antennasystem which responds tearriving waves i-n accordance with-some functionof the :angle between the direction 4of wave arrival `and the directionof maximum antennaresponse. lIt is characteristic of :conventionaldirective zantennas that -theresponse is relatively independent of vsaidangle when the angle is small. This results in a relatively Ibroad orindeterminate Abearing indication if the -antenna alignment providingmaximum signal pickup is used. On theother hand, the response changesrelatively rapidly with small Achanges, in angle near the position ofmum or zero pickup, providing 1a relativelynarrow angular rrange ofminimum pickup, i. e., a fshar-p null. Hence it is customary to operatedirection nders to provide `bearing indications with zero .or minimumantenna output, .although ,there `are .certain disadvantages to thistype of operation.

Among these vdisadvantages is the fact that the weaker the signalreceived, the broader becomes the null. Also, a false indication mayresult `from a temporary cessation of the signal, or from fading. Afurther and very important effect is that of noise and interference,which may mask the desired signal at angles near the null, causing anapparent broadening thereof.

Accordingly, it is an object of this invention to provide an improvedmethod of and means for direction nding, wherein bearings are .takenwhile the antenna output is near its maximum.

Another object is to provide an improved system for direction finding,employing two separate antennas with overlapping directive patterns andmeans for indicating equality of the responses of said antennas.

These and other objects will become apparent to those skilled in the artupon consideration of the following description with reference to theaccompanying drawing, of which Fig. 1 is a schematic block diagram of anembodiment of the invention, and Fig. 2 is a graph illustrating thedirectional response of the system of Fig. 1.

Referring to Fig. 1, two antenna elements I and 3 are mounted in areflector 5 in such positions that the directive patterns of the twoantennas are not coincident, but do overlap each other. The antennaassembly, including the reector 5, may be mounted in any suitablemanner, not shown, so as to be rotatable about a vertical axis. Theantenna EI is connected'to .a Yheterodyne detector or mixer 1, and theantenna 3 is yconnected to a similar mixer l1. A heterodyne .oscillator.I-I is connected in the .same way to both mixers] and .9. vAlowfrequency oscillator -,I.3, providing .output at, for example, 500.cycles Y per second, is connected in phase opposition to the .mixers 1and 9 4so yas to modulate their outputs. The -outputs of the mixers '1.and .9 .are supplied together to a variable gain intermedi-atefrequency amplier I5. The amplier I5 is in turn connected to 'a detecterand vaudio .amplifier I1. The loutput of the Adevice I1 is suppliedthrough a high pass filter .I9 zand an amplifier 2l toa voltageamplitude responsive indicator 23. The indicator 2.3 may be `of thevisual type, such as a meter, the aural type, such as a loudspeaker `ora ,pair of headphones, .or in fact any voltage :amplitude responsivedevice.

The output of the device -I1 is also applied through a lo-w pass filter.25 to a rectifier 21. The output Vof the -rectier .21 is applied to thegain control circuit of the intermediate frequency amplifier I5 so as.to vary the gain as .an :inverse function of the magnitude of theoutput of the rectiner 21.

The adjustment and operation of the .system .is as follows.: The outputs-of the antennas :I zand 3 are heterodyned vin the mixers 1 and 9respectively with the output of the oscillator =I.I, providing signalsat an intermediate frequency which are :proportional -i-n theiramplitudes to the respective antennafout-puts. The outputs of the.oscillator 'I3 to the mixers 1 and 9 are 'in opposite .polarities, andare adjusted to .a -level such that the mixers are allowed to operate.alternate-1y, each eperatingduringapproximately one-fourthzcf eachcycleof the oscillator I3.

When the two antenna outputs are equal, the combined output from themixers 1 and 9 appears in the intermediate frequency amplifier I5 with1000 cycle modulation. When the two antenna outputs are unequal therewill be in addition a 500 cycle modulation component. The modulatedintermediate frequency output of the amplifier I5 is detected andamplied by the device I1, and the modulation components are separated bythe filters I9 and 25. The 500 cycle component is rectified by thedevice 21, and controls the gain of the amplifier I5.

Thus if the antenna assembly is pointed in the general direction of atransmitter to which the system is tuned, some 1000 cycle energy will besupplied to the indicator 23, but the gain of the amplifier I5 will beheld down to a low or moderfier l likewise goes to zero, providing asharp increase in the amplitude of the 1000 cycle voltage applied to theindicator 2S. This is illusdetecting said. modulated and combinedsignals; controlling the extent of said amplification in accordance withthe amplitude of one of said amplified modulation components, andindicating said equality of amplitudes as a function of the amplitude ofthe other of said amplified components.

3. A radio direction finder including means for deriving from a radiowave two signals differently related in amplitude to the azimuth ofarrival of said wave, common oscillator means for modtrated lby the peakin the directional response curve of Fig. 2.

Thus the invention has been described as a system which providesextremely sharp directional indications at substantially the maximumresponse of a directive antenna system. This 4is accomplished by the useof two antennas arranged which overlap along a desired referencedirection. The outputs 0f the two antennas are modulated and Vcombinedso as to provide one modulation component related to theirV sum and asecond modulation component related to their diierence in amplitude. Thesecond component is then used to control inversely the magnitude of theiirst, providing a great increase in amplitude of the sum component uponequality of the two antenna outputs.

I Vclaim as my invention:

1. The method of determining the azimuth of arrival of anelectromagnetic wave comprising the steps of deriving from said wavetwoy signals diierently related in their amplitudes to the angle betweenthe line of arrival of said wave and a reference direction, modulatingsaid signals in opposite modulation phases, combining said modulatedsignals so as to produce a first modulation component related inamplitude to the sum of the amplitudes of said signals and a secondmodulation component related in amplitude to the diierence of theamplitudes of said signals, amplifying said combined signals, separatingsaid amplified modulation components, controlling the amplification ofsaid combined signals in response to the magnitude of one of saidamplified components, and determining said azimuth as a function of themagnitude of the other of said amplified modulation components.

2. The method of indicating equality of the amplitudes of two signals,comprising the steps of modulating said signals in opposite phases,combining said modulated signals to provide two modulation componentsrelated in amplitude respectively to the sum and to the difference ofthe amplitudes of said signals, amplifying and ulating said signalsseparately and in opposite phases, means for combining said modulatedsignals to produce different modulation components related in amplitudeto the sum and to the difference respectively of the amplitudes of saidsignals, and means for varying the amplitude of one of Ysaid componentsin response to variations in the amplitude of the other of saidcomponents.

4. A radiodirection nder including two directive antennas so positionedwith respect to each other as to provide overlapping response patterns,an oscillator, means for modulating the outputs of said antennas inopposite modulation phases with the output of said oscillator, anlvamplifier ararnged to amplify the modulated outputs of both of saidantennas, a detector connected to said amplifier, lter means arranged toseparate components in the output of said detector having the frequencyof said oscillator from components having twice the frequency of saidoscillator, indicator means arranged to respond to said lattercomponents, and rectifier means arranged to derive from said formeroomponents a voltage for controlling the gain oi' said amplifier.

5. A radio direction nder comprising two directive antennas positionedto provide overlapping response patterns, common oscillator means formodulating the outputs of said antennas in opposite modulation phases,and means for combining said outputs so as to provide two differentmodulation components, indicator means responsive to one of saidcomponents, and means responsive to the other of said components tocontrol the amplitudes of both of said components.

' CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,061,737 Offenhauser Nov. 24,1936 2,226,366 Braden Dec. 24, 1940

